Dc voltage regulator and impedance converter

ABSTRACT

A voltage regulator and impedance converter is interposed in the connections between a DC power supply and an intercom system having common communication and power circuits. The regulator and converter comprises a series connection of a resistor and an amplifier, preferably two transistors connected in a Darlington configuration. The amplifier regulates the DC output voltage under control of a source reference diode which is coupled to the amplifier&#39;&#39;s control electrode through a pair of limiting diodes and a second resistor. The amplifier is connected as an emitter follower, and the output voltage is regulated at the reference voltage of the reference diode minus the base-emitter voltage drop of the amplifier&#39;&#39;s transistors. Output transient voltage swings are limited to the forward drops of the limiting diodes. A connection is also provided from the output to the amplifier&#39;&#39;s control electrode through a capacitor. The RC time constant of the second capacitor and the second resistor connected to the amplifier&#39;&#39;s control electrode provides an output impedance which is low at DC and relatively high at voice frequencies, allowing the use of many communication sets on the common communication and power circuits.

United States Patent Dalke A i I a [54] DC VOLTAGE REGULATOR AND IMPEDANCE CONVERTER [72'] lnventori James A. Dalke, Seattle, Wash.

[73] Assignee: MetroData Corpration, Seattle,

Wash.

abandoned.

[52] US. Cl. ..323/22 T, 179/174, 307/30 [51] Int. Cl. ..G05f H04m 1/74, H02j [58] Fieldoi Search.....323/9, 22 T; 330/11 P, 204 P, 330/177,. 180; 179/78-80, 172-174; 307/1,

[ 1 Dec. 5, 1972 Primary ExaminerWilliam l-l. Beha, .lr. Attorney-Christensen & Sanborn [57] ABSTRACT A voltage regulator and impedance converter is interposed in the connections between a DC power supply and an intercom system having common communication and power circuits. The regulator and converter comprises a series connection of a resistor and an amplifier, preferably two transistors connected in a Darlington configuration. The amplifier regulates the DC output voltage under control of a source reference diode which is coupled to the amplifiers control electrode through a pair of limiting diodesand a second resistor. The amplifier is connected as an emitter follower, and the output voltage is regulated at the reference voltage of the reference diode minus the base-emitter voltage drop of the amplifiers transistors.

30 Output transient voltage swings are limited to the forward drops of the limiting diodes. A connection is also [56] References Cited provided from the output to the amplifiers control UNITED STATES pATENTS electrode through a capacitor. The RC time constant of the second capacitor and the second resistor con- 2,210,956 8/1940 Sk111man ..307/30 X nected to the amplifiers control electrode provides an 3,513,378 5/1970 Kemper ..323/22 X output impedance which is low at DC and relatively 3,214,668 10/1965 I Brmster ..323/22X high atvoice frequencies, allowing the use of many 2,301,346 7/ 1957 Rmlge" et UX communication sets on the common communication 3,471,794 l0/l969 Gugliotti -330/204 P and power circuim 3,310,731 3/1967 Ostroff et al ..323/22 X 12 Claims, 4 Drawing Figures x T: 90 WA .51 N 5 -9 f 3/ V1 11 1 DC VOLTAGE REGULATOR AND IMPEDANCE CONVERTER CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of U.S. Ser. No. 878,284, filed Nov. 20, 1969, and now abandoned entitled DC VOLTAGE REGULATOR AND IM- PEDANCE CONVERTER, inventor, JAMES A. DALKE. I

BACKGROUND OF THE INVENTION This invention generally relates to telephone-type insets are connected to abattery source through an inductor to prevent the audio signals from being attenuated by the power supply. The telephone sets may be connected together for intercommunication by direct connection. However, since each set presents a load to the power supply, the increased current demand caused by increasing the number of sets across the line results in a loss .of power voltage as a result of the resistance in the inductor.

' Designers of such systems avoid'the problem of having to place an unwieldy inductor in series with the supply by providing each set with a relatively small inductor, often termed a retard coil, which'is connected between the DC-supply and the set. The voice intercommunications may be accomplished, by tying the units together through a capacitor to a common bus.

It follows that a large number. of retard. coils-and capacitors are then required to connect a system of any appreciable size.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a means for connecting a plurality of telephone sets together using a common voice communication and power supply circuit.

It is a further object of this invention to provide interconnection of these telephone sets with a single power supply eliminating the use of multiple retard coils, and additionally furnishing voltage transient limiting means maintaining a constant average current to each set regardless of the number of sets used.

These objects and others which will be realized from a consideration of the following specification are attained, briefly, by replacing all of the conventional retard coils with a single voltage regulator and impedance converter, which comprises an amplifier providing voltage regulation with transient limiting and having a relatively high AC and low DC output impedance.

DESCRIPTION OF THE DRAWINGS taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagram of a prior art intercommunication system.

FIG. 2 is a diagram of an intercommunication system utilizing the voltage regulator and impedance converter.

FIG. 3 is a schematic diagram of the voltage regulator and impedance converter; and

FIG. 4 is a graph of the output impedance characteri'stic of the circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, illustrated therein is a communication system including, for purpose of explanation only, two communication stations 10 and 12 which are interconnected by a communication system generically designated 14. It is to be understood that any number of additional communication stations may be used with the intercommunication system 14. Each station includes at least one telephone set which is connected to the intercommunication system 14 by suitable means hereinafter to be described. If more'than one set isprovided in each station, provision also may be made for direct or switched communication'between the sets of the communication station. In the embodimentillustrated in FIG'. 1, communication station 10 includes three such telephone sets 16, 18, and 20. Each set includes an amplifier 22, a telephone transmitter 24 and a telephone receiver 26 which are conventional in configuration and interconnection. Power for the communication stations 10 and 12 is supplied via the intercommunication system 14 by means of a battery 30, one of whose terminals is connected to ground or reference potential and which serves as a common bus 31- and theother of whose terminals provides a DC voltage for the communication stations and which serves as a bus 32. Common bus 31 isconnected to eachof the telephone sets 16, I8 and 20 of communi-' cation station 10. Bus 32 is connected to'sets l6 and 20 through a retard coil 34 and a voltage dropping resistor 36.

By such an arrangement, the users of telephone sets 16 and 20 may constantly communicate in apoint-topoint fashion and the retard coil 34 and voltage dropping resistor 36 provide isolation of the voice circuit from the DC supply 30 so as to reduce any loading effects thereof. If communication is desired, however, with telephone set 18, the communication signals from either of the sets 16 or 20 must be connected together.

Set 18 is provided with DC potential from bus 32 by means of a retard coil 38 and a voltage dropping resistor 40. In FIG. I, the separation of telephone set 18 from set 16 and 20 allows those sets to be isolated and yet use a common power supply. However, the voice or communication signals must be transmitted by means of separate communication lines to effect complete voice isolation. To this end sets 16 and 20 are connected through a capacitor 42 to one input terminal 43 of a switching network 44 and set 18 is connected through a capacitor 45 to another input terminal 26. Network 44 may comprise a simple switch arrangement with the requisite number of poles and movable contacts. Alternatively, switching network 44 may comprise any network having capability to interconnect any combination of communication lines. Connected to the output lines of switching network 44 are, for purposes of illustration only, three communications lines 48, 50 and 52. Line 48 is connected to bus 59 and uses that busto transmit voice signals. Likewise, line 50 is connected to a communication line 58, and line 52 to a communication line 57.

Lines 31, 32, 57, 58 and 59 form the basis of the intercommunication system 14 and are thus available for connection to communication station 12 and any additional stations desired. Communication station 12 may be a duplicate of station 10, as illustrated in FIG. 1, or it may have any configuration required. If station 12 is a duplicate of station 10, it should be noted that required thereinare two retard coils 34' and 38'. In'operation, the user of set 16, for example, may directly communicate with the user of set at station 10. When communication is desired with set 18, switching network 44 is set so that connection is made between the input terminals 43 and 46 thereof. Sets 16 and 18 may also be interconnected through the output lines 48, 50 and 52. When operating in this mode, communication may also be made between sets 16 and 18 and any of the telephone sets of station.l2 connected to the chosen output line by the switching network 44 of station 12. j

In any intercommunication system wherein paths for DC voltage for the intercommunication units and the voice signals therebetween are provided along common lines, it is evident that certain characteristics are desired of a successful design. Because of these common circuits, there is a necessity that separation be made of the voice signals from the DC supply voltage. In FIG. 1 this is accomplished by feeding the voice signals through capacitors 42 and 45 to the switching network 44 which interconnects them and other voice signals in a suitable manner.

In using the interconnection scheme in the prior art just described, it is evident that for maximum flexibility in connecting the telephone sets together a separate DC supply bus 32 must be provided throughout the system. Another problem which exists in prior art is voltage transients which result when the telephone sets are switched from one interconnecting bus to another as a result of current changes through the inductors.

Therefore, systems such as illustrated in FIG. 1, although approximating desired performance, require a great deal of additional components, such as retard coils, the voltage dropping resistors and the large isolation capacitors, to those nominally used. These components merely approximate the desired operating characteristics of an intercommunication system having a plurality of independently usable telephone sets.

For purposes of economy and performance, it would be desirable to provide or accomplish all these functions by means of a single circuit or a very few circuits. Such a circuit is the object of this invention and is illustrated by the diagrams in FIGS. 2 and 3.

.As noted by comparison of FIGS. 1 and 2, retard coils such as 34 and 38, voltage dropping resistors 36 and 40, and capacitors 42 and 45 have been eliminated from box 10, and replaced by circuits 60, 62 and 64 which are interposed between the DC supply and the three communications lines 66, 68 and 70. Each unit 60, 62, and 64 comprises a voltage regulator and impedance converter which forms the subject matter of this invention. If simple point-to-point communication is desired between telephone sets 16, 18, and 20 of communication station 10 and those associated with communication station 12 and any other stations, only unit 60 and communication line 66 and common bus 31 would be required.

A complete understanding of how the units 60, 62, and 64 effectively replace the heretofore used retard coils can best be understood by reference to the schematic diagram of FIG. 3. The DC voltage supply is illustrated as a battery 30 at the left hand side of the figure. Lead 32 in this embodiment connects the positive terminal of the battery 30 to a first common junction point 7.2 through a voltage dropping resistor 74. Point 72 is connected to output point 76 by means of an amplifier 77 connected as an emitter follower. Amplifier 77 may comprise two transistors 78 and 80 in a standard Darlington connection wherein the emitter of transistor 80 serves as the output point 76 and the base of transistor 78 serves as the control electrode for the amplifier 77. The DC input voltage present at point 72 is connected to a second common junction 82 by means of a resistor 84. Connected between point 82 and the negative terminal 31 is a means for providing a constant reference voltage which includes the parallel connection of a capacitor 86 and a reference or zener diode 88. The constant reference potential thus obtained at point 82 is coupled to the control electrode of the DC amplifier, or the base electrode of transistor 78, by means of a parallel connection of a resistor 90 and two reverse-parallel diodes 92, and 94 whose limiting function will be hereinafter described. The output voltage present at point 76 is likewise connected to the control electrode by means of a circuit including a resistor 96 connected across the output and a capacitor 98.

In operation, the amplifier 77 including the Darlington transistors 78 and 80 operates at essentially a unity voltage gain. Control of the output voltage and current is accomplished by varying the bias point thereof under control of the voltage regulation means and what may be termed an impedance conversion section. The desired output voltage is set by choosing the reference or zener diode 88. This reference voltage is maintained across the capacitor 86 and appears at point 82. If for any reason the output voltage at point 76, should vary from the reference voltage at point 82, this voltage change will result in current being conducted through resistor 90 to the base or control electrode of amplifier 77 to vary the gain thereof in a manner such as to compensate for the output voltage variation. Operation of such a circuit is standard in the art and needs no further description. However, the use of a two transistors in Darlington for amplifier 77 merits special consideration, for it allows the use of a high value resistor 90 and hence allows amplifier 77 to have a characteristically high input impedance. This forms one portion of the impedance conversion section hereinafter to be described.

As noted above, diodes 92 and 94 perform a transient limiting function. Although in the embodiment of FIG. 2 the conventional retard coils have been eliminated, and thus the inductance presented to the power supply has been reduced somewhat, still there is considerable inductance in the telephone sets. .When one of these sets is added to or removed from the line, the current therethrough tends to stay constant and the voltage across the set changes rapidly and produces a spike which istransmitted along the line. Normally the current through resistor 90 is so small the voltage drop is somewhat less than the forward conduction voltage of diode 92 and 9 4, and hence the diodes are not conducting. However, when the output voltage swings positive an amount greater than the forward conduction of diode 92, diode 92 conducts to provide a cur rent paththrough diode 88 to common bus 31 to clamp the base electrode of transistor 78 at the referencevoltage plus the forward voltage drop of diode 92. A similar operation occurs when the output voltage drops, an amount below the reference voltage equal to the forward conduction voltage of diode 94. At such time, diode 94 conducts to discharge capacitor 86 and thereafter clamp the base electrode 78 at the reference voltage minus the forward voltage drop of diode 94.

Capacitor 98 blocks the DC output voltage but couples any AC voltage components directly to the control electrode of the amplifier. Capacitor 98 and the resistor 90 form an RC circuit whose time constant is such a'sflto provide a desired rising'impedance characteristic with frequency. 'l he characteristic may be appropriately chosen so as to be approximately zero at DC and to sharply rise at frequencies in the audio or voice communication range. An experimentally obtainedoutput impedance characteristic of one embodiment of this circuit is illustrated in FIG. 4. I Referring back now to FIG. 2, it is quite evident that considerablecost saving has been effected since only three circuits of the nature illustrated in FIG. 3 are required for three communication lines. Because of the output impedance characteristics of circuits 60, 62 and 64, each of the lines 66,68, and 70 may supply a virtually unlimited number of communication stations such as stationsl0 and 12.

Whilethis invention has been particularly described in terms of a single embodiment, as illustrated in FIG. 3, it is to be clearly understood by those skilled in the art the invention is in no way limited thereto. For example, the pair of transistors 78, 80 connected in Darlington may be replaced by any other stable, amplifier operating at a unity voltage gain..Likewise, voltage dropping resistor 74 may not be required in all cases. Accordingly, the scope of the invention is to be measured only by the limits of the appended claims.

What I claim is:

1. An improved intercommunications system comprising:

a. a plurality of voice communication andpower circuits,

b. at least one intercommunication means coupled to each of said plurality of voice communication and power circuits, each of said intercommunication means including a transmitter and a receiver,

c. means providing selective interconnection paths between said voice communication and power circuits,

- d. a source of DC supply voltage,

- e. 'a plurality of voltage regulation and impedance conversion circuits, each of said circuits coupling 6.energy storage means connected between said output terminal and said control terminal, the values of said energy storage means and said resistance means being chosen so as to provide an impedance at said output terminal which is low in DC and which gradually'increases to a.relatively high value at AC frequencies in the voice range. i l

2. A system as recited in claim 1, wherein each of said voltage regulation and impedance conversion circuits includes first and second diodes connected in reverse-parallel between said reference voltage source and said control terminal of said amplifier.

3. An improved intercommunications system as recited in claim 1, wherein said amplifier comprises first and second transistors, the base of said first transistor being said control terminal, the emitter of said second transistor being said output terminal, the collectors of said first and second transistors being coupled together and being said input terminal, and the emitter of said first transistor being connected to the base of said second transistor.

4. An improved intercommunications system comprising: I

a. a common voice communications and power circuit, I t

b. a plurality of intercommunications means coupled to said common voice communications and power circuit, each of said intercommunications means including a transmitter and a receiver,

c. a source of DC supply voltage, I H

d. a voltage regulation and impedance conversion circuit, said circuit coupling said source of DC supply voltage to said common voice communica tions and power circuit and including:

i. an amplifier having input, output and control terminals ii. reference voltage means supplying a DC signal to said control terminal for controlling the voltage on said output terminal, and

iii. feedback means coupled between said output terminal for coupling to said control terminal only those portions of any AC signals appearing on the output terminal thereof which have frequencies within the voice range for controlling the impedance at said output terminal.

5. An improved intercommunications system as recited in claim 4, wherein said amplifier comprises first and second transistors, the base of said. first transistor being said control terminal, the emitter of said second transistor being said output terminal,v the collectors of said first and second transistors being coupied together and being said input terminal, and the emitter of said first transistor being connected to the base of said second transistor.

6. An improved intercommunications system as recited inclaim 4, wherein said feedback means of said voltage regulation and impedance conversion circuit comprises a capacitor connected from the output terminal of said amplifier therein to said control terminal, and wherein said reference voltage means of .said circuit comprises a zenerdiode coupled to the source of the DC supply voltage, and further comprising a resistance means coupling said zener diode to said control terminal, the values of said capacitor and said resistance means being chosen so as to provide an impedance at saidoutput terminal which is low at DC and which graduallyincreases to a relatively high value at AC frequencies in the voice range.

' 7. An improved intercommunications system as recited in claim 6, wherein said voltage regulation and impedance conversion circuit includes first and second diodes connected in reverse-parallel between said zener diode and said control terminal of said amplifier.

8. In an intercommunications system comprising a common communications and power circuit and a source of DC supply voltage, a new use for a voltage regulation and impedance conversion circuit, said circuit comprising an'amplifier having input, output and control terminals, reference voltage means for supply of a DC signal to said control terminal for controlling the voltage on said output terminal, and feedback means coupled between said output terminal for coupling to said control terminal only those portions of any AC signals appearing on the output terminal thereof which have frequencies within the voice range for controlling the impedance at said output terminal, said new use comprising the application of said circuit for isolating the source of DC supply voltage from the common communications and power circuit by applying the DC supply voltage to the amplifiers input terminal and by coupling signals present on said amplifiers output terminal to the common communica terminal, and wherein said reference voltage means of source. of the DC supply voltage, and further comprisinga resistance means coupling said zener diode to said control terminal, wherein said new use further comprises the step of choosing the values of said capacitor and said resistance means so as to provide an impedance at said output terminal which is low at DC and which gradually increases to a relatively high value at AC frequencies in the voice range.

10. A new use for an intercommunications system as recited in claim 8, wherein said circuit further includes first and second diodes connected in reverse-parallel between said zener diode in said control terminal of said amplifier. v

11. A method for coupling a DC supply-voltage from a source thereof through an amplifier having input, output and control. terminals to a common communications and power circuit, while yet isolating any intercommunications means connected to said common communications and power circuit from said source of DC supply voltage, said method comprising the steps a. supplying said DC supply voltage to said amplifiers input terminal, coupling an output voltage appearing on said amplifiers output terminal to said common communications and power circuit,

. regulating said output voltage by supplying a reference voltage to said control terminal of said amplifier, and

. providing an impedance at said amplifiers output terminal which is low at DC and which gradually increases to a relatively higher value at AC frequencies within the voice range by coupling to said control terminal only those portions of any Ac signals appearing on the output terminal of said amplifier which have frequencies within the voice range.

12. A method as recited in claim 11, further including the step of limiting the output voltage on said amplifiers output terminal by applying said reference voltage directly to said amplifiers control terminal whenever said output voltage exceeds said reference voltage by a predetermined amount. 

1. An improved intercommunications system comprising: a. a plurality of voice communication and power circuits, b. at least one intercommunication means coupled to each of said plurality of voice communication and power circuits, each of said intercommunication means including a transmitter and a receiver, c. means providing selective interconnection paths between said voice communication and power circuits, d. a source of DC supply voltage, e. a plurality of voltage regulation and impedance conversion circuits, each of said circuits coupling said source of DC supply voltage to one of said voice communication and power circuits and including
 1. an amplifier having input, output and control terminals,
 2. means coupling the source of DC supply voltage to said input terminal,
 3. means coupling the communication and power circuit to said output terminal,
 4. means deriving from said source of DC supply voltage a reference voltage,
 5. resistance means coupling said reference voltage to said control terminal, and
 6. energy storage means connected between said output terminal and said control terminal, the values of said energy storage means and said resistance means being chosen so as to provide an impedance at said output terminal which is low in DC and which gradually increases to a relatively high value at AC frequencies in the voice range.
 2. means coupling the source of DC supply voltage to said input terminal,
 2. A system as recited in claim 1, wherein each of said voltage regulation and impedance conversion circuits includes first and second diodes connected in reverse-parallel between said reference voltage source and said control terminal of said amplifier.
 3. means coupling the communication and power circuit to said output terminal,
 3. An improved intercommunications system as recited in claim 1, wherein said amplifier comprises first and second transistors, the base of said first transistor being said control terminal, the emitter of said second transistor being said output terminal, the collectors of said first and second transistors being coupled together and being said input terminal, and the emitter of said first transistor being connected to the base of said second transistor.
 4. An improved intercommunications system comprising: a. a common voice communications and power circuit, b. a plurality of intercommunications means coupled to said common voice communications and power circuit, each of said intercommunications means including a transmitter and a receiver, c. a source of DC supply voltage, d. a voltage regulation and impedance conversion circuit, said circuit coupling said souRce of DC supply voltage to said common voice communications and power circuit and including: i. an amplifier having input, output and control terminals ii. reference voltage means supplying a DC signal to said control terminal for controlling the voltage on said output terminal, and iii. feedback means coupled between said output terminal for coupling to said control terminal only those portions of any AC signals appearing on the output terminal thereof which have frequencies within the voice range for controlling the impedance at said output terminal.
 4. means deriving from said source of DC supply voltage a reference voltage,
 5. resistance means coupling said reference voltage to said control terminal, and
 5. An improved intercommunications system as recited in claim 4, wherein said amplifier comprises first and second transistors, the base of said first transistor being said control terminal, the emitter of said second transistor being said output terminal, the collectors of said first and second transistors being coupled together and being said input terminal, and the emitter of said first transistor being connected to the base of said second transistor.
 6. An improved intercommunications system as recited in claim 4, wherein said feedback means of said voltage regulation and impedance conversion circuit comprises a capacitor connected from the output terminal of said amplifier therein to said control terminal, and wherein said reference voltage means of said circuit comprises a zener diode coupled to the source of the DC supply voltage, and further comprising a resistance means coupling said zener diode to said control terminal, the values of said capacitor and said resistance means being chosen so as to provide an impedance at said output terminal which is low at DC and which gradually increases to a relatively high value at AC frequencies in the voice range.
 6. energy storage means connected between said output terminal and said control terminal, the values of said energy storage means and said resistance means being chosen so as to provide an impedance at said output terminal which is low in DC and which gradually increases to a relatively high value at AC frequencies in the voice range.
 7. An improved intercommunications system as recited in claim 6, wherein said voltage regulation and impedance conversion circuit includes first and second diodes connected in reverse-parallel between said zener diode and said control terminal of said amplifier.
 8. In an intercommunications system comprising a common communications and power circuit and a source of DC supply voltage, a new use for a voltage regulation and impedance conversion circuit, said circuit comprising an amplifier having input, output and control terminals, reference voltage means for supply of a DC signal to said control terminal for controlling the voltage on said output terminal, and feedback means coupled between said output terminal for coupling to said control terminal only those portions of any AC signals appearing on the output terminal thereof which have frequencies within the voice range for controlling the impedance at said output terminal, said new use comprising the application of said circuit for isolating the source of DC supply voltage from the common communications and power circuit by applying the DC supply voltage to the amplifier''s input terminal and by coupling signals present on said amplifier''s output terminal to the common communications and power circuit.
 9. A new use for an intercommunications system as recited in claim 8, wherein said feedback means of said circuit comprises a capacitor connected from the output terminal of said amplifier therein to said control terminal, and wherein said reference voltage means of said circuit comprises a zener diode coupled to the source of the DC supply voltage, and further comprising a resistance means coupling said zener diode to said control terminal, wherein said new use further comprises the step of choosing the values of said capacitor and said resistance means so as to provide an impedance at said output terminal which is low at DC and which gradually increases to a relatively high value at AC frequencies in the voice range.
 10. A new use for an intercommunications system as recited in claim 8, wherein said circuit further includes first and second diodes connected in reverse-parallel between said zener diode in said control terminal of said amplifier.
 11. A method for coupling a DC supply voltage from a source thereof through an amplifier having input, output and control terminals to a common communications and power circuit, while yet isolating any intercommunications means connected to said common communications and power circuit from said source of DC supply voltage, said method comprising the steps of: a. supplying said DC supply voltage to said amplifier''s input terminal, b. coupling an output voltage appearing on said amplifier''s output terminal to said common communications and power circuit, c. regulating said output voltage by supplying a reference voltage to said control terminal of said amplifier, and d. providing an impedance at said amplifier''s output terminal which is low at DC and which gradually increases to a relatively higher value at AC frequencies within the voice range by coupling to said control terminal only those portions of any Ac signals appearing on the output terminal of said amplifier which have frequencies within the voice range.
 12. A method as recited in claim 11, further including the step of limiting the output voltage on said amplifier''s output terminal by applying said reference voltage directly to said amplifier''s control terminal whenever said output voltage exceeds said reference voltage by a predetermined amount. 